In the practice of such processes liquid feedstocks have been disintegrated into discrete droplets by one of two techniques. The first technique entails injecting the feedstock below the freezant surface. In this technique, the injection orifice is normally immersed in the freezant. See, for example, U.S. Pat. No. 3,484,946. This technology suffers from the disadvantage that a carefully heated injection orifice is required to prevent freezing of liquid in the immersed orifice with resultant plugging of the orifice by frozen feedstock. An attendant disadvantage is that once started the process cannot be conveniently stopped or interrupted: When the flow of liquid through the orifice stops, the residual liquid in the orifice will freeze at once. Thus this frozen material must be removed from the orifice before operations can resume. The heated orifice may also denature labile constituents in the liquid product. Finally, an undesirable proportion of fine product particles can result from the violent boiling of freezant at the interface between the heated orifice and the freezant.
The second and most widely adopted technique is denominated herein by the term "spray freezing." Typical spray freezing processes are exemplified in U.S. Pat. Nos. 3,228,838, 3,721,725, 3,928,566 and 3,932,943. In these processes, the liquid feedstock is atomized or formed into droplets prior to entering the freezant. However, we have found this method to be unsatisfactory where future uses or further processing of the lyophilized particles requires a low proportion of fine particles.
This is particularly the case with lyophilized biological fluids to be used as controls, standards or calibrators for various analytical or diagnostic test procedures, or for the instruments used to perform such test procedures. These products as a group will hereinafter be referred to as "quality control materials" and, where such materials are derived from a blood fraction, they are henceforth defined as "quanlity control plasma." Quality control plasma is intended to include serum, plasma, as well as defibrinated plasma. Aside from the problem of aerosol formation of potentially infective fluids, quality control materials prepared by spray freezing suffer from at least three deficiencies. These deficiencies are largely a function of the high proportion of fine particles in the lyophilized product. The term "fine particles" is used hereinafter to mean those particles which are capable of passing through United States Standard Mesh No. 20 as that mesh is ordinarily used to determine particle size. Stated differently, these are particles which have at least one cross-sectional dimension smaller than about 850 microns.
The first deficiency experienced with spray frozen product is that the fine particles acquire a static charge, particularly under the conditions of low humidity in which lyophilized substances are stored. The particles tend to adhere to one another and to the walls of their containers, thus making handling quite difficult. This difficulty in handling becomes especially significant during production of quality control materials.
If spray frozen, lyophilized biological fluids are to be employed as quality control materials they must be precisely weighed out into containers such as 10, 25 or 50 ml. vials, a process that will hereinafter be referred to as a "weigh and fill operation." One suitable device for accomplishing such weigh and fill operations is disclosed in U.S. Pat. No. 2,701,703. Other such devices are well known to those skilled in the art. The ordinary difficulty of rapidly and precisely weighing very small masses into containers with automatic equipment is exacerbated by electrostatically charged product, and fine particles create the most difficulty. The charged fine particles, because of their low mass, cling to the surfaces of the equipment and to one another or larger particles. This impedes the free flow of the material and continuously varies the container fill rates, often bringing containers over their weight tolerance limits. These limits are quite narrow in the case of quality control materials: If variable amounts of the lyophilized control are weighed into vials of the same lot, reconstitution in constant amounts of aqueous solution will yield a constituent variation that is directly proportional to the variation in control material from vial to vial. This is particularly undesirable with reconstituted standards.
Standards are generally biological fluids containing stated constituent concentrations. They are assayed by the laboratory using its reagents and instruments, and the results plotted against the manufacturer's stated concentrations of the assayed constituent. This plot is then used for a predetermined period to arrive at constituent levels for all samples tested. If this plot is in error because the actual constituent levels are different from the manufacturer's reported concentrations, the reported results for every patient sample compared with the plot would be in error as well.
Other liquid feedstocks which are to be formed into particles, lyophilized and weighed into containers are equally susceptible to severe quality control problems. For example, pharmaceutical dosages and diagnostic reagents require tolerances equally close to those of quality control materials. Here again, a large proportion of fine product particles makes it extremely difficult to achieve rapid, uniform, automated dispensing of product by weight.
The deficiencies of spray frozen liquid feedstocks do not end with variable container fills and concomitant high container rejection rates. Even if a container is filled with a mass of quality control material within the established weight tolerance it may nonetheless be completely unsatisfactory. This raises the second deficiency of quality control materials prepared by conventional spray freezing techniques: The concentrations of clinically significant constituents are not uniform over the entire range of particle sizes. For example, fine particles of human control serum may contain up to 10% less creatine phosphokinase activity than does the lyophilized material as a whole. Thus even if a container is filled to the proper weight it may contain a greater proportion of one particle size than other containers in the same lot of feedstock. Fine particles, for example, may predominate during the later portion of a filling operation. In such case the last vials of any control serum lot will exhibit artificially low creatine phosphokinase activity. Variation in constituent levels is as deleterious as variable fill levels, yet it is completely impractical to detect and reject such defective containers by individually assaying all of the containers in the lot.
The third deficiency of spray freezing stems from the required removal and disposal of excessive fine particles from the spray frozen compositions. Removing such particles from spray frozen quality control materials, for example by sieving, increases the electrostatic charge on the remaining fine particles. Sieving also alters constituent concentration in the final product as compared to the starting material because, as discussed above, constituent levels are not constant over the entire range of particle sizes. Finally, the fine particles removed from the spray frozen product are either waste or must be recycled through the process again, thereby at least doubling the processing costs for that proportion of product.
While it would be desirable to reduce the production of fine particles ab initio, it is equally important to avoid the formation of "large particles," i.e., those capable of being retained by United States Standard Mesh No. 12 at that mesh is ordinarily used to determine particle size. Stated differently, these large particles have at least one cross-sectional dimension larger than about 1650 microns. The disadvantage of large particles is that they tend to fracture, thus generating more unwanted fine particles as well as jagged fragments which impede the free flow of the particle mass.
It is therefore an object of this invention to produce a lyophilized product which can be accurately, rapidly and conveniently weighed from bulk lots into a plurality of uniform portions.
It is an additional object of this invention to provide a process and apparatus for freezing particles of a liquid feedstock wherein the flow of feedstock can be interrupted without additional processing disruptions, without high capital requirements or complex equipment, and without risking the destruction of labile constituents of the product.
It is another object of this invention to produce a frozen particulate composition in which the proportion of fine particles is significantly reduced without concomitantly increasing the proportion of large particles.
It is a further object of this invention to provide a process and apparatus for freezing particles of biological fluid which, when lyophilized, can be accurately and rapidly distributed by automatic weigh and fill devices from bulk lots into a plurality of uniform portions.
It is a still further object of this invention to produce a quality control material which is substantially homogeneous with respect to the concentrations of its constituents and which can be employed in weigh and fill systems without resulting in excessive rejection rates of filled containers for overfilling.
These and other objects of this invention will be apparent to those skilled in the art from a consideration of this specification taken in its entirety.